One commonly used form of electrical fuseholder assembly includes a socket-forming body designed to be mounted on a panel so as to accept insertion of fuses from the front of the panel. The fuse is carried and held in place by a knob of one form or another which fits into an opening on the front of the body. Frequently such knobs have a bayonet-type retention lock, i.e., a retention lock configured with locking members so that the knob is first axially inserted to a certain depth into the body opening against the force of a fuse-retaining spring and then rotated to a position so that upon removal of the insertion pressure the retaining spring forces the locking members into locking engagement. Removal of the knob is achieved by first pressing axially inward on it to separate the locking members, then rotating the knob in a direction opposite to the locking direction, and then releasing the pressure thereon, whereupon the retaining spring force ejects the knob from the body.
Such fuseholder assemblies permit rapid insertion of the fuse-carrying knob as compared with alternative types wherein screw threads are provided on the knob and body. The former type of fuseholder assembly is shown in the patent literature, as for example, in U.S. Pat. No. 4,448,476 issued May 15, 1984 to Perlman et al. Such assemblies typically include a conducting sleeve coaxially mounted in the fuseholder knob and carrying an axial spring mounted therewithin, the socket-forming body having an electrical side terminal contact which is configured to lockingly engage with the sleeve so as to provide not only the mechanical locking action, but also electrical contact from the side terminal to the outer end of an inserted fuse.
Such fuseholder assemblies typically exhibit a variety of drawbacks. First, the use of the two metallic structures to form the locking means and the electrical contact as well can give rise to undesirably high contact resistance between the locking members. This arises from the fact that, particularly in the case of high amperage fuses, fuseholders may run rather hot. Since there are normally many plastic insulating portions in the fuseholder assembly, the result is that the engaging surfaces of the locking members may become contaminated by organic compounds volatilized from the plastic. Additionally, such locking means are fragile, and if the knob is not fully inserted prior to rotation, portions of the lock member elements may be improperly engaged after rotation of the knob and become deformed so as to become inoperative.
An additional problem arises in such fuseholder assemblies in that they tend to be unstable in the event of incomplete final rotation of the knob during insertion, i.e., if the knob is not sufficiently rotated that the fuse-retaining spring is able to move the knob slightly outward to the locking position, then subsequent vibration may allow the fuse knob to rotate to a point where the knob is undesirably ejected.
In addition to the instability associated with the use of conventional bayonet lock means to retain the fuse knob in the fuseholder body, a further problem is commonly experienced when the fuseholder knob has a screwdriver slot head which, when the knob is in place and finally positioned within the fuseholder body, does not project beyond the outer surface of the fuseholder body (or would not do so in the absence of spring pressure). The knob can be removed by inserting a screwdriver into the slot in the knob head and rotating it into its unlocked position where spring pressure forces the knob outward where it can be grasped and removed from the fuseholder body. However, it is not an uncommon experience for an operator to extract a suspect fuse, find that it is blown, and place the knob back in the holder temporarily while he goes off to find a replacement fuse. At this point the customary knob spring pressure bearing on the fuse, which normally will eject the knob in the released orientation thereof as described above, is missing. Thus, upon attempting to remove the knob with the spring in this relaxed condition by rotating the knob with a screwdriver, the user finds that the knob remains depressed within the fuseholder body where he cannot grasp it. A sharply pointed knife or the like is then needed to pry or pull the knob out of the fuseholder body.
A similar situation is also encountered in those cases where the pressure spring is carried within a conducting sleeve mounted as an extension of the knob assembly, and wherein the fuse end cap accommodated therein has become so corroded so as to freeze to the sleeve. Under such conditions the spring force cannot act against the fuse to assist in the extraction of the knob.
Finally, it is well known that there exist a great variety of fuses manufactured in cartridges of identical diameter, but of different length. If a fuse of accidentally oversize length is inserted into a conventional axial fuseholder, then upon forcing the fuse knob inward to retain the fuse, the axial spring may easily be stressed to the point of permanent deformation. This results in reduced axial seating pressure when a fuse of proper length is installed thereafter. This can cause a fuseholder to run hot at the terminals because of inadequate seating pressure to maintain the contact.